A series of comparative (i.e., multispecies) experiments to analyze the elicitation and expression of sexually proceptive behavior in female animals. Proceptive actions are those by which the female assumes initiative in instigating and directing the progress of the mating process. Proceptive is contrasted to receptive behavior which comprises those responses the female must perform in permitting the male to execute his copulatory pattern. The basis of the estrous female's sexual attraction to males will be investigated by systematic study of different sensory modalities (e.g. vision, olfaction, audition). Other experiments will examine the possibility that a female's attraction to particular male may be influenced by previous association, so that males reared with a female are more (or perhaps less) attractive than unfamiliar males. Effects of previous mating experience with a male on his subsequent attractiveness will be studied. Other experiments deal with the relative effects of prolonged copulatory activity on the female's proceptive and receptive behavior toward a male. Whereas male animals usually lose sexual responsiveness and potency after one or several ejaculations, females appear capable of maintaining their receptivity even after mating with several males in succession. Two objectives are to measure the duration of receptivity in females exposed to continuous sexual stimulation, and to observe possible changes in proceptive behavior associated with progressive increase in total amount of copulatory stimulation. A separate study on prenatally masculinized female dogs will determine whether such animals exhibit normal neuroendocrine function as reflected in cyclic secretion of ovarian hormones accompanied by ovulation.